In plug-in hybrid electric vehicles (PHEVs), the fuel vapor canister primarily adsorbs refueling vapors, as refueling and diurnal vapors are sealed within the fuel tank by a fuel tank isolation valve. An air intake system hydrocarbon (AIS HC) trap may capture hydrocarbons emitted by leaky injectors for from fuel that may puddle in intake. The AIS HC trap may also capture uncombusted fuel that is trapped within the engine cylinders themselves. An AIS trap is required for vehicles to be classified as practically zero emissions vehicles (PZEVs).
However, depending on the position of the cylinder intake and exhaust valves when the engine is shut off, the uncombusted fuel may migrate to either the engine intake or the exhaust manifold and may then escape to atmosphere. This may both increase a vehicle's emissions and cause a vehicle to fail emissions certification testing.
Previous solutions to this problem involve the utilization of secondary air injection methods to reduce the escape of uncombusted and partially combusted hydrocarbons to atmosphere at a subsequent vehicle-on event. For example, U.S. Pat. No. 6,971,357 describes a method wherein a secondary air pump provides fresh air to scavenge exhaust from engine cylinders upon engine shutdown. However, secondary air injection adds additional hardware and complexity to the vehicle.
In one example, a method for an engine is provided, comprising following an engine-off event, positioning a first engine cylinder with an intake valve open and an exhaust valve open and purging contents of the first engine cylinder to an exhaust catalyst. In this way, existing vehicle hardware may be used to evacuate residual hydrocarbons and uncombusted fuel to the exhaust catalyst, provided the catalyst remains above the light-off temperature. As one example, a reversible vacuum pump coupled within an evaporative leak check module may be used to pressurize the engine intake when a canister purge valve is opened. This may further allow fuel vapor stored in a fuel vapor canister to be purged to the exhaust catalyst, decreasing bleed emissions without requiring an additional bleed canister coupled between the fuel vapor canister and atmosphere.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.